Fire crews tackling a major blaze at a world-famous art school said they have prevented the destruction of the building's structure and the majority of its contents.

Flames engulfed Glasgow School of Art's (GSA) Charles Rennie Mackintosh building at around 12.30pm, with firefighters battling all day to save one of Scotland's most famous structures.

The blaze has now been brought under control after what has been described as a "very black day" for the institution.

The fire started during a busy time as students worked to complete their end-of-year assessments but everybody was evacuated from the building safely.

The institution is one of Europe's leading art schools, while the listed Mackintosh-designed building is a tourist attraction in its own right.

Construction work started on the building in 1897 after Mackintosh won a design competition while working for Glasgow architects Honeyman & Keppie. It took around 10 years to complete due to funding issues and several alterations were made by Mackintosh during the process.

More than a century later, it still operates as a working building serving hundreds of students.

Indications are the firefighters' efforts today have ensured more than 90% of the structure is "viable" and up to 70% of contents protected, according to the Scottish Fire and Rescue Service.

Assistant Chief Officer Dave Boyle said: "Crews have been working absolutely flat out throughout this very challenging incident and it is clear their effort and skill has saved this treasured building and many of the items it housed.

"While the priority from the outset was to save life we have also been working closely with Glasgow School of Art staff to ensure firefighters conducted an effective salvage operation.

"We are of course very conscious the Mackintosh is a world renowned building that is a key feature of this great city, and that the artworks it stores are not only valuable but also cherished."

Crews will remain at the scene throughout the night dampening down hot spots, a spokeswoman for the fire service said.

Flames could be seen through the scorched windows and billowing out of the roof of the sandstone building on the corner of the city's Renfrew Street and Scott Street as the fire took hold.

Large crowds gathered near the scene as multiple fire crews tackled the blaze, with several people crying as they watched the events unfold.

Broadcaster Muriel Gray, a former student and current chairwoman of the school, was among those who burst into tears when she saw the building in flames.

She said: "It's been an absolutely devastating day for everyone involved with the School of Art, students who are preparing for their degree show and all the staff.

"We just want to make it clear that we are so grateful to the fire service who responded within four minutes and there were no casualties which is the main thing that we were concerned about.

"It's a very black day and we have a lot of things to do and think about now."

Mr Boyle added: "Work to save everything that can be saved is ongoing and we will continue to work closely with GSA staff and students throughout this operation."

It is hoped that many of the students' work can be saved.

Mr Boyle said: "We are acutely aware this period is the culmination of years of endeavour for students and that their irreplaceable work is inside the Mackintosh."

A GSA spokeswoman added: "We would like to express our very sincere thanks to the Scottish Fire and Rescue Service for their tremendous efforts throughout today."

The fire prompted an outpouring of concern online from those connected to the school, or who hold affection for it.

Scotland's First Minister Alex Salmond said on Twitter that his thoughts were with staff and students.

Fran Healy, who studied at GSA before becoming lead singer of Travis, expressed his sadness.

The singer tweeted: "Just super super sad this place which has been with Glasgow and with all the students is burning down..."

The Royal Institute of British Architects (RIBA) tonight described the blaze as "an international tragedy".

"The most important work by Charles Rennie Mackintosh, an architect of international significance, Glasgow School of Art is held in the highest regard by architects and the public alike - it was rightly judged to be the best building of the past 175 years, in a nationwide poll run by the Royal Institute of British Architects.

"Damage to a building of such immense significance and uniqueness is an international tragedy. It is irreplaceable."

Alex Misick, 25, was helping his girlfriend put the finishing touches to her degree show project across the road in the school's new Reid building when they saw plumes of smoke.

The GSA graduate, now vice president of the Students' Association, said: "I think everyone is in shock. It's soul-destroying.

"It was literally a few hours to go before everyone finished up and handed in. That's everyone's work over three or four years. Everyone's in tears."

Earlier, chief officer Alasdair Hay, of the fire service, said: "We regularly attend significant fires but the iconic status of this building is not lost on us.

"Part of our operation is to try and salvage what we can and we worked with colleagues from the school who identified objects of significance that they would like us, if possible, to save.

"They described the objects and their location and we briefed officers and firefighters where they could, did the salvage if it was safe to do so."

He could not say what objects were saved.

He added: "There are structural engineers here and we are working with them to save as much of the fabric as we possibly can."

In an earlier statement, the fire service said crews reached the scene within four minutes of receiving the first 999 calls.

The service said on its Twitter account tonight: " Thank you for all messages of support on the @GSofA fire today. Still much work to be done. We know how much the building means to people."